In a fuel cell system, fuel gas such as hydrogen and oxidant gas containing oxygen electrochemically react with each other via electrolyte to take out an electric energy from electrodes disposed on both sides of the electrolyte. Especially, a solid polymer fuel cell employing a solid electrolyte attracts public attention as an electric power supply for an electric vehicle because of a low operating temperature and ease of handling. A fuel cell powered vehicle is an ultimate clean vehicle with only water remaining as emission matter. This vehicle is installed with a hydrogen absorbing device, such as a high pressure hydrogen tank, a liquid hydrogen tank, and a hydrogen absorbing amorphous alloy tank. Hydrogen supplied from the hydrogen absorbing device and air including oxygen are delivered to the fuel cell to accomplish reaction for taking out the electric energy from the fuel cell to drive a motor connected to drive wheels.
The solid polymer fuel cell includes an anode off-gas recirculation type that anode off-gas containing non-reacted hydrogen gas discharged from an anode outlet is recirculated to the anode inlet. This type fuel cell includes an anode (fuel electrode) inlet supplied with hydrogen at a larger flow rate than that of hydrogen consumed in electrochemical reaction with anode off-gas.
When such an anode off-gas recirculation type fuel cell is continually operated, nitrogen oxide concentration and steam concentration in anode gas increase because air and steam are leaked from a cathode (air electrode). Further, if hydrogen forming fuel gas contains impurities, they accumulate in anode gas without being consumed in electrochemical reaction unlike hydrogen.
These phenomena undesirably degrade an electric power generating performance. Hence, after operation has been continued for a time interval to some extent, anode off-gas containing a large amount of impurities is discharged from the anode off-gas recirculation unit to an external combustor in which combustion takes place between anode off-gas and air.
With such a combustor proposed by Japanese patent Application Laid-Open No. 8-78030, the flow rate of air is controlled such that the temperature of the combustor is controlled not to exceed a heat-resistant temperature.
With this technology, the fuel cell system includes a fuel reformer formed with a combustion chamber into which portions of anode off-gas and cathode off-gas are introduced and combusted; an air compressor driven with a cathode exhaust gas turbine; and a low temperature blower that allows air, together with air delivered from the air compressor, to be drawn into the combustion chamber.
Further, this fuel cell system includes a controller provided for outputting a rotating speed command to the low temperature blower while outputting an opening-degree correction command to a reformer-combustion-air flow-rate regulator valve, based on an output command related to the fuel cell, an oxygen concentration of combustion exhaust gas, and an outlet temperature of the combustor. If the outlet temperature of the combustion chamber exceeds an upper limit, the controller controls the opening degree of the reformer-combustion-air flow-rate regulator valve to be increased and the rotating speed of the low temperature blower to be increased in dependence on the resulting opening degree for increasing the flow rate of air to be supplied to the combustion chamber of the reformer to lower the temperature of the combustion chamber.